A time-of-flight mass spectrometer (TOF-MS) includes an introduction unit, a TOF unit, a gain adjuster, an ion extraction signal generator and a data collecting circuit, and analyzes a component contained in the sample by ionizing the sample, accelerating the ionized sample, causing the ion of the sample to fly, and measuring a flying time and intensity (voltage value) of the ion on the basis of a mass of the ion.
In analysis performed by the TOF-MS, the sample to be analyzed is first ionized by the introduction unit and then sent into the TOF unit at the same time as the measurement start time. A voltage is applied to the ion introduced in the introduction unit when an ion extraction signal is provided, so that the ion flies along a predetermined orbit in the TOF unit that is in a vacuum state.
When the ion reaches (collides with) a detector included in the TOF unit, the detector outputs a detected ion signal. The amplitude of the detected ion signal is adjusted by a gain adjuster or the like and collected by a data collecting circuit that includes an A/D converter. The data collected by the data collecting circuit is output to an input/output device through a CPU. Results of the measurement are displayed as a mass spectrum. The component contained in the sample can be analyzed on the basis of intensities (voltage values) of the spectrum and times (masses) corresponding to the intensities.
Normally, for the TOF-MS, measurement sensitivity (S/N ratio) of spectrum data obtained by a single measurement is not sufficient in many cases. Therefore, the measurement is performed plural times and an accumulation process is performed so as to obtain a mass spectrum and improve the measurement sensitivity.
Hereinafter, a measurement to be performed to obtain a mass spectrum is called a mass spectrum measurement, and a single measurement is called TOF scan. The TOF scan is to collect data about ion accelerated in accordance with a single ion acceleration signal, with the data output from the detector, that is, to collect spectrum data for a time period from a time t0 (when the ion is accelerated) to a time t1.
In recent years, there has been a demand for a data collecting circuit, which uses an A/D converter to obtain such a mass spectrum as described above, to have a high dynamic range in order to improve mass spectrometry.
This is due to the fact that there has been a demand for the aforementioned TOF scan to perform detection of one to several hundred ions simultaneously during one scan time. Thus, it is difficult to detect a signal while ensuring a desired signal-to-noise (SN) ratio within an input range for a single A/D converter. As a method for avoiding this, for example, Patent Literature 1 (JP-A-2005-268152) discloses “a data processing device for mass spectrometry, which is included in a time-of-flight mass spectrometer, the processing device including: an A/D converter that samples a detected signal; a first determining circuit that performs a level determination operation on data sampled by the A/D converter using a predetermined threshold so as to divide the data into two components; an accumulation memory that stores data whose levels are equal to or higher than the threshold while performing an accumulation process on the data; and a second determining circuit that accumulates data whose levels are lower than the threshold, performs the level determination operation on the accumulated value using the threshold, and causes data whose levels are equal to or higher than the threshold to be stored in the accumulation memory”.
In addition, for a signal detected by a photomultiplier known as a photometer, since the intensity of the signal varies depending on a wavelength of light, it is difficult for a single A/D converter to detect a signal in some cases. As a method for avoiding this, for example, Patent Literature 2 (JP-A-H09-166491) discloses “an interferogram processing method in which an interferogram that is output from an analyzer is simultaneously sampled by plural gain amplifiers that use gains that are different from each other, percentages of data series of the gains which are different from each other are gradually changed at around the time when the gains are switched during a combination of the sampled data”.